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A COMPARISON OF MONITORING TECHNIQUES FORIMPROVED EROSION CONTROL: A FIELD STUDY

Bill HedgesBP Trinidad & Tobago

PO Box 714Port of Spain

Trinidad & Tobago

Andy BodingtonConcepts & Services Co. Ltd.

PO Box 1336Port of Spain

Trinidad & Tobago

ABSTRACT

Erosion, corrosion and erosion-corrosion problems provide significant safety andenvironmental risks to the oil and gas industry due to unexpected material failure. In additionthe cost associated with such failures is estimated to be many millions of dollars each year dueto deferred or lost production and repair costs. Severe damage has occurred to tubing,flowlines, pipe fittings, headers, valves, pumps, and other production equipment. In somecases projects choose expensive corrosion resistant alloys (CRA’s) to mitigate againsterosion-corrosion. For the gas fields discussed in this paper an erosion management strategyis in place but two major problems remain:

1. Multiple flowlines (up to three) have been used on each high production rate well (up to250 mmscf/d). This creates space issues that have resulted in the need for unusualpiping routes and platform extensions. In addition all equipment and control systems

are multiplied accordingly.2. Rates are restricted to values based on the API 14E equation. Currently field

production is ~1.5 bscf/d of gas. A 1% increase in velocity could result in an increase of 15 mmscf/d.

Consequently this semi-quantitative project was sanctioned to evaluate a range of monitoringequipment to determine if it might be possible to safely increase production rates.



INTRODUCTION

Erosion, corrosion and erosion-corrosion problems provide significant health, safety andenvironmental risks to the oil and gas industry due to unexpected material failure. In additionthe cost associated with such failures is estimated to be many millions of dollars each year dueto deferred or lost production and repair costs.

Severe damage has occurred to tubing, flowlines, pipe fittings, headers, valves, pumps, and

other production equipment. In some cases projects choose expensive corrosion resistantalloys (CRA’s) to mitigate against erosion-corrosion.

To date, the most effective counter-measures against erosion and corrosion involve usingerosion-corrosion resistant alloys, inhibitors, coatings or placing limitations on production andflow velocities. In the latter case the oil and gas industry often use guidelines based on the

API 14E standard1. Unfortunately this guideline was developed for the erosion of steel bywater droplets in steam and does not recognize many of the important factors contributing toerosion and corrosion damage. These include the presence of solid particles contained in theflow, corrosivity of the fluid, formation and removal of corrosion scale, geometries of the fittingsand flow regime.

In Trinidad, BP has an erosion management strategy that can be summarised as follows:

1. Do not produce solids (sand).This is implemented at the design stage of a well. The geologists and reservoir engineer determine how consolidated the reservoir is and then design the well completionaccordingly. For highly consolidated sands this may require nothing to be done. For lessconsolidated sands a range of measures including gravel packs and sand screens areused.

2. Assume that a nominal mass of sand will be produced.

This is a practical approach that reflects the known limitations of the sand control measuresnoted in (1) above. At this time the company’s guidelines2 define nominally sand freeconditions as ≤0.1 pounds of sand for every million standard cubic feet of gas (≤0.1lbs/mmscf).

3. Limit the velocity of the fluids to a maximum value to avoid erosion. As a default the API 14E guidelines are used in which the maximum velocity allowed (Vmax)is set equal the erosional velocity (Verosion). This is the velocity above which unacceptablerates of erosion are expected to occur and is calculated using the following equation:

Vmax = Verosion = C (1)

√gas density

‘C’ is a constant (known as the C factor), the value of which is determined by thecompany’s central technology center and is dependent on the operating conditions (e.g.flow regime, presence of corrosion inhibition) and material of construction of the pipework.

At this time2 the value for Carbon steel systems with good corrosion inhibition is 135. For multiphase mixtures the mixture density replaces the gas density.



When the API 14E calculation yields a result that is unacceptably low or when more data isprovided (sand size, shape and production rate) a more detailed calculation of the erosionalvelocity is made. These are done in collaboration with experts in the company’s technologyGroup (EPTG). The models used are developed through research and collaborativeprograms, such as those at Harwell3, UK and Tulsa University4, U.S.A.

4. Monitor the gas (fluids) to ensure that sand rates do not exceed a nominal value.Currently the nominal value is set to 0.1 lbs/mmscf and the monitoring is undertaken using

acoustic sand detectors located at strategic points on the flowline (e.g. bends). These areindividually calibrated using known quantities of sand of known size and shape. Oncecalibrated, a maximum signal from the detector is assigned and if this is exceededproduction is choked back until the signal falls below the assigned value. Production isthen slowly increased back to the desired rate.

5. Frequent inspection of high risk locations. A simple risk based approach is used to identify the most vulnerable locations. Typicallythese are the chokes and flowlines at bends. Chokes are opened and inspected visually. If sand is found it is collected and analyzed for size distribution. A typical inspection intervalis 3 months.

To date this approach has proved reasonably successful in controlling erosion and erosioncorrosion. However it has several limitations, which are:

• The API 14E calculation was developed for steam applications without solids. Clearlyits use for multiphase flow (gas, oil, water) with solids is an extrapolation outside itsoriginal design. In some cases this leads to an overly conservative design (high alloysteels or restrictive flow rates) whilst in others it underestimates the severity of theproblem.

• The focus on a nominal sand rate is problematic for two reasons. First, it assumes thesand is produced continuously at the same rate and, in practice, this is often not the

case with sand produced intermittently. In some cases little or no sand is produced for long periods of time followed by periods of very high rates. Secondly, the actual rate of sand can be very difficult to determine in the field.

• The focus on a critical erosional velocity assumes that this can be calculated or determined accurately.

• The acoustic monitors have proven very successful for detecting sand. However theyhave not been successful in determining the quantity of sand. Moreover, they do notmeasure the damage that the sand causes.

These limitations have led to two major problems, which are:

1. Multiple flowlines (up to three) have been used on each high production rate well (up to 250mmscf/d). This creates space issues that have resulted in the need for unusual pipingroutes and platform extensions. In addition all equipment and control systems aremultiplied accordingly.

2. Rates are restricted to the API 14E levels. Currently field production is ~1.5 bscf/d of gas. A 1% increase in velocity could result in an increase of 15 mmscf/d.



Consequently this semi-quantitative project was sanctioned to evaluate a range of monitoringequipment to determine if it might be possible to safely increase production rates.

EXPERIMENTAL

Choice of Flowlines

For this project a gas production platform was chosen which met the following criteria:

• Availability of multiphase (oil, water & gas) and gas only wells.

• The flow rates of the two flowlines should be easily adjustable.

• Availability of suitable injection points for the injection of sand.

• No issues with the installation of any of the instruments.

Two separate flowlines were chosen to meet these criteria. These were numbered 1A and 7.

Flow velocity and flow regime calculations

Tables 1 and 2 detail the conditions for the flowlines 1A and 7.

TABLE 1Physical Data For Flowline Number 1A.(Nominal Diameter = 6”, Schedule 80)

(OD = 168 mm (6.626”), ID = 146 mm (5.761”), WT =11 mm (0.432”)

ChokePosition

(Open %)

Gas(mmscf/d)

Oil(bbls/d)

Water (bbls/d)

Pressure(psia)

Temp.(°F)

Vsg

(m/s)Vso

(m/s)Vsw

(m/s)

73 20 160 4 1050 120 5.60 0.02 0.000481 31 302 7 1070 120 8.52 0.03 0.0008

90 47 302 7 1070 120 12.92 0.03 0.0008

100 56 366 15 1086 120 15.16 0.04 0.0020

TABLE 2

Physical Data For Flowline Number 7.(Nominal Diameter = 6”, Schedule 80)

(OD = 168 mm (6.626”), ID = 146 mm (5.761”), WT =11 mm (0.432”)

ChokePosition(Open %)

Gas(mmscf/d) Oil(bbls/d) Water (bbls/d) Pressure(psia) Temp.(°F) Vsg(m/s) Vso

(m/s) Vsw(m/s)

66 39 1,484 526 1,110 120 10.21 0.16 0.06

75 40 1334 535 1090 120 10.80 0.15 0.06

81 47 1400 560 1121 120 12.33 0.15 0.06

In all cases the flow regimes were calculated to be annular.



Monitoring Equipment

Following a survey of available monitoring equipment, six (6) instruments were selected for evaluation in the field and these are listed in Table 3.

TABLE 3Monitoring Instruments Evaluated

Instrument Supplier MeasurementTechnique

Installation Characteristics

AcousticSand

Detector

Roxar Acoustic –detects solids

NonIntrusive,clamp on

type

Single instrument. Listensfor solids impacting oninternal surface of flowline.Can be calibrated tomeasure quantity of sand if flow conditions remainrelatively constant.

Ultrasonic

SandDetector

A Research

Tool Stillunder Development

Ultrasonic –

detects solids

Non

Intrusive,clamp ontype

Sends pulsing sound waves

through flowline wall, aswell as through flowingmedium in flowline andreceives reflected signal.Change in reflected signal,indicates solids presence inflowing fluid.

Flexible UTMat

BodycoteMetal

Technology

Ultrasonic -Measures wall

thickness

NonIntrusive,

bonded onflowline

with epoxy

Flexible mat with 14individual sensors eachmeasuring wall thickness bypulse echo technique. Mats

are permanently installed inposition.

HighSensitivityUltrasonic

Probe

A ResearchTool Still

under Development


thickness

NonIntrusive,clamp on

type

Single instrument usingpulse echo technique.Sends sound wavesthrough flowline wall onlyand receives reflectedsignal. Claimed to be verysensitive.

ConventionalUltrasonic

Probe

Krautkramer Ultrasonic

Systems


thickness

Nonintrusive

hand held

Regular UT probe, whichmeasures the flowline’s wall

thickness.High

Sensitivity ERProbe

CormonLimited

Ceiontechnology -Measuresmetal loss

rate.

Intrusivevia access

fitting

Angled head type installeddirectly in flow stream.Element metal loss relativeto reference elementcauses change in electricalresistance.



In all cases the representatives of the suppliers were contracted to install the equipment toensure that there were no concerns relating to this aspect of the trial. Further details of theinstruments can be obtained from the respective manufacturers or the authors of this paper.

Location Of The Probes On The Flowlines Wherever possible the probes were placed at a location identified as vulnerable to erosion, usua

just after a bend. Figures 1 & 2 show schematically the placements of all the probes on the tw

flowlines.

Figure 1Probe Arrangement on Flowl ine 1A

Figure 2Probe Arrangement on Flowline 7

Flow Flow

Legend:

UT flexible mats

High Sensitivity ER probe

High Sensitivity UT probe

Acoustic Sand detector

Conventional UT probe

Ultrasonic sand detector



Sand Injection Equipment

A sand injector was used to inject known quantities of sand of known size into the flowlines.The injector consisted of a tank, which was filled with a suspension of sand in a viscous gel, ahigh pressure hose with a series of ball valves and a check valve. One end of the hose wasconnected to the tank and the other end to an injection point on the flowline upstream of theinstruments. The third component of the injection unit consisted of a pneumatic panel, whichhoused the controls, gauges and a pump.

The system, when pressured up has a higher pressure than the line pressure and the sandmixture is injected into the flowline at a continuous rate. Due to the limited volume of the tank,injection was only possible for 5 to 10 minutes, the exact duration being dependent on the flowrate. After all the sand from the tank was injected, the injection process was be stopped, thetank refilled and injection resumed.

Figure 3 shows a typical plot of injection rate versus time.

FIGURE 3

Typical Sand Injection Profile

Sand injected at a rate of 2g/s

No sand injected. Tank beingrefilled

To enable the sand to be pumped and to ensure that the sand concentration was as uniformas possible the sand was mixed into a polymer gel to form a suspension. The suspension wasformed by mixing a very small amount of the gel, approximately 250 ml in a bucket filled withapproximately 6 litres of water. This was mixed together with a high-speed air driven mixer

until the fluid started to get more viscous. The weighed quantity of sand was then placed intothe bucket and the high-speed mixer used to mix the sand and gel. The mixture was thenpoured into the tank of the injector unit. The sand was injected at a known mass per second.Using the known gas production rate this sand rate was converted to a sand loading in poundsof sand per million standard cubic feet of gas (lbs/mmscf).



Experimental Objectives

The two primary objectives were to determine:

1. Which of the two sand detectors, if any, could detect small quantities of sand of differentsizes (80-120 μm and 30-50 μm) at different flow velocities. The systems’ reliability,response time, repeatability and user friendliness were also observed.

2. Which of the metal loss measuring instruments or combination of instruments, if any,

could detect small changes in metal loss due to sand injection.

It should be noted that this was a broad study involving numerous experiments repeatedseveral times and the full report consists of ~150 pages of data. This paper is focused on theoverall conclusions and so many of the experimental results are omitted.

RESULTS

Sand Detection Investigation

The data reported here is for flowline 7 under the following conditions:

Choke settings: 75%Pressure: 1090 psi

Sand size: 80-120 μmInjection rate: 2g/sSand Loading: 9.5 lbs/mmscf

Ultrasonic Sand Detector

Figure 4 shows the oscilloscope trace of the detector during the experiments with an

explanation of the various components of the signal.



FIGURE 4Ultrasonic Sand Detector Results

MIST's sand d etector signal

-100

-80

-60

-40

-20

0

20

40

60

80

100

0 100 200 300 400 500 600 700 800 900 1000

Time, ms

S i g n a l , %

High sensitivity Ultrasonic probe

No signal obtained

UT sand detector

Flowline I.D.

Black arrow is reflected signalfrom other side of wall internal.The reflected signal passesthrough the flowing medium.No signal observed.

Blue arrow isreflectedsignal frominside wall

Green arrow isreflectedsignal frominside wall

Flowline O.D.

The manufacturer claims that if sand is present a third reflection will be seen in the circledarea. However, at no time was such a signal seen in any of the tests in this program despitethe manufacturers personnel making three separate visits to service and calibrate the

equipment. It was therefore concluded that the ultrasonic sand detection equipment was not capable of detecting the sand under the conditions of these tests.



Acoustic sand detector

Figure 5 shows the sand injection profile used and the corresponding response of the acousticdetector.

FIGURE 5Sand Injection Profi le And Corresponding Acoust ic Sand Detector Response

Roxar's Sand Trend

0

100000

200000

300000

400000

500000

600000

700000

2 8 / 0 4 / 2 0 0 3

1 1 : 5 6 : 1 7

2 8 / 0 4 / 2 0 0 3

1 2 : 0 3 : 2 4

2 8 / 0 4 / 2 0 0 3

1 2 : 1 0 : 3 0

2 8 / 0 4 / 2 0 0 3

1 2 : 1 7 : 3 7

2 8 / 0 4 / 2 0 0 3

1 2 : 2 4 : 4 4

2 8 / 0 4 / 2 0 0 3

1 2 : 3 1 : 5 0

2 8 / 0 4 / 2 0 0 3

1 2 : 3 8 : 5 7

2 8 / 0 4 / 2 0 0 3

1 2 : 4 6 : 0 4

2 8 / 0 4 / 2 0 0 3

1 2 : 5 3 : 1 0

2 8 / 0 4 / 2 0 0 3

1 3 : 0 0 : 1 7

2 8 / 0 4 / 2 0 0 3

1 3 : 0 7 : 2 3

2 8 / 0 4 / 2 0 0 3

1 3 : 1 4 : 3 0

2 8 / 0 4 / 2 0 0 3

1 3 : 2 1 : 3 7

2 8 / 0 4 / 2 0 0 3

1 3 : 2 8 : 4 3

2 8 / 0 4 / 2 0 0 3

1 3 : 3 5 : 5 0

2 8 / 0 4 / 2 0 0 3

1 3 : 4 2 : 5 6

2 8 / 0 4 / 2 0 0 3

1 3 : 5 0 : 0 3

2 8 / 0 4 / 2 0 0 3

1 3 : 5 7 : 1 0

2 8 / 0 4 / 2 0 0 3

1 4 : 0 4 : 1 6

2 8 / 0 4 / 2 0 0 3

1 4 : 1 1 : 2 3

2 8 / 0 4 / 2 0 0 3

1 4 : 1 8 : 3 0

2 8 / 0 4 / 2 0 0 3

1 4 : 2 5 : 3 6

2 8 / 0 4 / 2 0 0 3

1 4 : 3 2 : 4 3

2 8 / 0 4 / 2 0 0 3

1 4 : 3 9 : 4 9

2 8 / 0 4 / 2 0 0 3

1 4 : 4 6 : 5 6

2 8 / 0 4 / 2 0 0 3

1 4 : 5 4 : 0 3

2 8 / 0 4 / 2 0 0 3

1 5 : 0 1 : 0 9

2 8 / 0 4 / 2 0 0 3

1 5 : 0 8 : 1 6

2 8 / 0 4 / 2 0 0 3

1 5 : 1 5 : 2 2

Time

S i g n a l , n a n o v o l t s

A torcoustic Sand Detec Flowline 7

Gel only

Figure 5 shows that the background signal prior to any injections was smooth and constant

with a value of 21,000 nV. Injection of the gel only increased the signal to 60,000 nV. Injectionof the sand in the gel gave a signal of ~200,000 nV. It is clear from the data that the acousticsand detector detected every injection of sand.

Limit Of Detection Of The Acoustic Sand Detector

The acoustic detector was shown to be reliable and repeatable for the detection of sand over awide range of conditions. It did however have some limits at either low velocities and/or

Backgroundnoise

Sand easily detected. Each peakrepresents when 2g/s was injected



concentrations. Figure 6 shows the data for Flowline 1A with a choke setting of 81% for decreasing concentrations of both 80-120 and 30-150 μm sand.

FIGURE 6 Acoust ic Sand Detector Response To Decreasing Sand Concentrations

Roxar sand tr end. Sand de tection limit analysis.

80-120 03

0

10002000

3000

40005000

60007000

8000

900010000

11000

1200013000

1400015000

16000

1700018000

19000

2000021000

22000

2300024000

2500026000

27000

2800029000

30000

Flowline 1A. Choke 81%

and 30-50 micr ons . May 12, 20

Time


80-120 μm 30-50 μm

Acoustic sand detector trend.Sand detector limit analysis for

80-120 and 30-50 microns sand

0.95g/s

1.4g/s or 8.6 lbs/mmscf

0.01g/s or 0.06 lbs/mmscf 0.06g/s

0.14g/s

Gel

0.27g/sOpen choke to81%

It was observed that for the 80-120 µm sand the lowest rate of sand detection was ~ 0.01g/s or 0.06 lbs/mmscf which is below the limit for nominally sand free conditions of 0.1 lbs/mmscf asstated in the company guidelines2.

For the 30-50 µm sized sand the lowest rate of sand detection was ~ 1.4 g/s or 8.6 lbs/mmscf which is much higher than the guide of 0.1 lbs/mmscf. Thus it is possible for fine sand to beproduced that is not detected by the acoustic detectors. An important question is, therefore,

whether this sand causes any significant damage to the flow line.

Metal Loss Investigations

Figure 7 shows the sand injection profile and the corresponding responses from the acousticsand detector and the 4 metal loss detectors located on flowline 1A. The conditions for thistest were: choke 100%, Vsg = 15.16 m/s, Vso = 0.04 m/s, sand size = 80-120 µm.



FIGURE 7Instrument Responses To 80-120 Microns Sized Sand

(Severe Conditions)

Roxar sand trend, Flowline 1A, CHOKE 100%, 80-120 microns

May 07, 2003

0

50000

100000

150000

200000

250000

300000

9 : 0 4 : 2 5

9 : 1 3 : 4 0

9 : 2 2 : 5 4

9 : 3 2 : 0 9

9 : 4 1 : 2 4

9 : 5 0 : 3 9

9 : 5 9 : 5 4

1 0 : 0 9 : 0 8

1 0 : 1 8 : 2 3

1 0 : 2 7 : 3 8

1 0 : 3 6 : 5 3

1 0 : 4 6 : 0 8

1 0 : 5 5 : 2 2

1 1 : 0 4 : 3 7

1 2 : 1 0 : 1 2

1 2 : 1 9 : 2 7

1 2 : 2 8 : 4 1

1 2 : 3 7 : 5 6

1 2 : 4 7 : 1 1

1 2 : 5 6 : 2 6

1 3 : 0 5 : 4 1

1 3 : 1 4 : 5 5

1 3 : 2 4 : 1 0

1 3 : 3 3 : 2 5

1 3 : 4 2 : 4 0

Time


MIST's wall t hickness measurements

0

1

2

3

4

5

6

7

8

9

10

11

9 : 0 6 : 0 0

9 : 1 5 : 0 0

9 : 2 1 : 0 0

9 : 2 7 : 0 0

9 : 3 3 : 0 0

9 : 4 2 : 0 0

9 : 5 1 : 0 0

9 : 5 7 : 0 0

1 0 : 0 4 : 0 0

1 0 : 0 9 : 5 5

1 0 : 1 5 : 0 0

1 0 : 2 4 : 0 0

1 0 : 3 3 : 0 0

1 0 : 4 2 : 0 0

1 0 : 5 1 : 0 0

1 1 : 0 0 : 0 0

1 1 : 0 9 : 0 0

1 1 : 1 5 : 0 0

1 1 : 2 2 : 0 0

1 1 : 3 0 : 0 0

1 1 : 3 3 : 0 0

1 1 : 4 2 : 0 0

1 1 : 5 1 : 0 0

1 2 : 0 0 : 0 0

1 2 : 0 9 : 0 0

1 2 : 1 7 : 0 0

1 2 : 2 4 : 0 0

1 2 : 3 3 : 0 0

1 2 : 4 2 : 0 0

1 2 : 4 9 : 0 0

1 2 : 5 6 : 0 0

1 3 : 0 3 : 0 0

1 3 : 1 2 : 0 0

1 3 : 2 1 : 0 0

1 3 : 2 7 : 0 0

1 3 : 3 3 : 0 0

1 3 : 4 2 : 4 0

Time

M e t a l L o s s , m m

Cormon's Real Time Metal loss t rend

Metal Loss vs Time

30035040045050055060065070075080085090095010001050110011501200125013001350140014501500155016001650170017501800185019001950200020502100215022002250230023502400

9 : 0 6 : 0 0

9 : 1 5 : 0 0

9 : 2 1 : 0 0

9 : 2 7 : 0 0

9 : 3 3 : 0 0

9 : 4 2 : 0 0

9 : 5 1 : 0 0

9 : 5 7 : 0 0

1 0 : 0 4 : 0 0

1 0 : 0 9 : 5 5

1 0 : 1 5 : 0 0

1 0 : 2 4 : 0 0

1 0 : 3 3 : 0 0

1 0 : 4 2 : 0 0

1 0 : 5 1 : 0 0

1 1 : 0 0 : 0 0

1 1 : 0 9 : 0 0

1 1 : 1 5 : 0 0

1 1 : 2 2 : 0 0

1 1 : 3 0 : 0 0

1 1 : 3 3 : 0 0

1 1 : 4 2 : 0 0

1 1 : 5 1 : 0 0

1 2 : 0 0 : 0 0

1 2 : 0 9 : 0 0

1 2 : 1 7 : 0 0

1 2 : 2 4 : 0 0

1 2 : 3 3 : 0 0

1 2 : 4 2 : 0 0

1 2 : 4 9 : 0 0

1 2 : 5 6 : 0 0

1 3 : 0 3 : 0 0

1 3 : 1 2 : 0 0

1 3 : 2 1 : 0 0

1 3 : 2 7 : 0 0

1 3 : 3 3 : 0 0

1 3 : 4 2 : 4 0

Time

M e t a l L o s s , n a n o m e t e r s

Krautkramer's wall thickness measurements

0.41

0.42

0.43

0.44

0.45

0.46

0.47

0.48

9 : 0 4 : 2 5

9 : 1 2 : 0 0

9 : 1 8 : 0 0

9 : 2 4 : 0 0

9 : 3 0 : 0 0

9 : 3 9 : 0 0

9 : 4 8 : 0 0

9 : 5 4 : 0 0

1 0 : 0 3 : 0 0

1 0 : 0 9 : 0 0

1 0 : 1 2 : 0 0

1 0 : 2 1 : 0 0

1 0 : 3 0 : 0 0

1 0 : 3 9 : 0 0

1 0 : 4 8 : 0 0

1 0 : 5 7 : 0 0

1 1 : 0 6 : 0 0

1 1 : 1 2 : 0 0

1 1 : 1 9 : 0 0

1 1 : 2 7 : 0 0

1 1 : 3 2 : 0 0

1 1 : 3 9 : 0 0

1 1 : 4 8 : 0 0

1 1 : 5 7 : 0 0

1 2 : 0 6 : 0 0

1 2 : 1 5 : 0 0

1 2 : 2 1 : 0 0

1 2 : 3 0 : 0 0

1 2 : 3 9 : 0 0

1 2 : 4 8 : 0 0

1 2 : 5 4 : 0 0

1 3 : 0 0 : 0 0

1 3 : 0 9 : 0 0

1 3 : 1 8 : 0 0

1 3 : 2 4 : 0 0

1 3 : 3 0 : 0 0

1 3 : 3 9 : 0 0

Time

W a l l t h i c k n e s s ,

i n c h e s

It is clear that the acoustic detector responded well to all of the sand injections with arepeatable signal. However these responses only indicate that solids are present (detection)but do not provide a measure of how much damage they might be doing.

The ER probe also responded well to all of the sand injections although there was significantvariation in the magnitude of the responses for the same conditions. The total metal loss

Right rax 's M1 wal l th i ckness measurements

0

1

2

3

4

5

6

7

8

9

10

11

12

9 : 0 6 : 0 0

9 : 1 5 : 0 0

9 : 2 1 : 0 0

9 : 2 7 : 0 0

9 : 3 3 : 0 0

9 : 4 2 : 0 0

9 : 5 1 : 0 0

9 : 5 7 : 0 0

1 0 : 0 4 : 0 0

1 0 : 0 9 : 5 5

1 0 : 1 5 : 0 0

1 0 : 2 4 : 0 0

1 0 : 3 3 : 0 0

1 0 : 4 2 : 0 0

1 0 : 5 1 : 0 0

1 1 : 0 0 : 0 0

1 1 : 0 9 : 0 0

1 1 : 1 5 : 0 0

1 1 : 2 2 : 0 0

1 1 : 3 0 : 0 0

1 1 : 3 3 : 0 0

1 1 : 4 2 : 0 0

1 1 : 5 1 : 0 0

1 2 : 0 0 : 0 0

1 2 : 0 9 : 0 0

1 2 : 1 7 : 0 0

1 2 : 2 4 : 0 0

1 2 : 3 3 : 0 0

1 2 : 4 2 : 0 0

1 2 : 4 9 : 0 0

1 2 : 5 6 : 0 0

1 3 : 0 3 : 0 0

1 3 : 1 2 : 0 0

1 3 : 2 1 : 0 0

1 3 : 2 7 : 0 0

1 3 : 3 3 : 0 0

1 3 : 4 2 : 4 0

T i m e

M e t a l L o s s , m m

No metal loss observedby the flexible UT mat

Acoustic sand detector trendSand easily detected

Continual computer hardwareproblems encountered with the

high sensitivity ultrasonic probe

No metal loss observed by the

conventional ultrasonic probeHigh sensitivity ER probe

Metal loss rate about 19 mm/yr



during this test was 2,010 nm (0.00201 mm). However, the total time of actual injection was55 minutes which equates to a metal loss rate of ~19 mm/y. This is clearly an unacceptablerate and this result illustrates the importance of not only detecting the presence of the solidsbut also being able to determine how much damage they are doing.

The flexible UT mat, sensitive and conventional UT probes did not detect anything duringthese tests. If the wall loss of 2,010 nm, as measured by the ER probe, is accurate this is notsurprising since the limit of detection of all of these instruments in significantly greater than this

and in the range of ~ ± 0.1 mm.

Figure 8 shows the sand injection profile and the corresponding responses from the acousticsand detector and the ER metal loss detector located on flowline 7 for less severe conditionsthan those in figure 7. The conditions for this test were: choke 75%, Vsg = 10.8 m/s, Vso = 0.21m/s, sand size = 30-50 µm. The results for the other metal loss instruments are not shownbecause, as seen above, they did not detect any metal loss during these tests.

FIGURE 8Instrument Responses To Smaller Sized Sand (30-50 Microns)

(Moderate Conditions)

Cormon's Real Time Metal Loss Trend

Metal Loss vs Time

02468

101214161820222426283032343638404244464850

1 0 : 5 6

1 1 : 0 9

1 1 : 1 8

1 1 : 3 0

1 1 : 3 7

1 1 : 3 9

1 1 : 4 5

1 1 : 5 3

1 2 : 0 0

1 2 : 1 5

1 2 : 3 0

1 2 : 4 5

1 2 : 5 6

1 3 : 0 3

1 3 : 1 2

1 3 : 2 1

1 3 : 2 9

1 3 : 3 6

1 3 : 4 3

1 3 : 5 6

1 4 : 1 5

1 4 : 2 5

1 4 : 4 0

1 4 : 5 6

1 5 : 1 2

1 5 : 2 0

1 5 : 5 1

1 5 : 5 8

Time

M e t a l L o s s / n a n o m e t e r s

High sensitivity ER Probe

Metal loss rate of 0.26 mm/y

Roxar sand trend

30-50 microns s and injected Flowline 7, 75% chok e

0

20000

40000

60000

80000

100000

120000

140000

2 9 / 0 4 / 2 0 0 3 1

0 : 5 6 : 0 1

2 9 / 0 4 / 2 0 0 3 1

5 : 0 9 : 4 5

2 9 / 0 4 / 2 0 0 3 1

5 : 1 9 : 4 1

2 9 / 0 4 / 2 0 0 3 1

5 : 2 9 : 3 7

2 9 / 0 4 / 2 0 0 3 1

1 : 3 5 : 4 1

2 9 / 0 4 / 2 0 0 3 1

1 : 4 5 : 3 6

2 9 / 0 4 / 2 0 0 3 1

1 : 5 5 : 3 1

2 9 / 0 4 / 2 0 0 3 1

2 : 0 5 : 2 6

2 9 / 0 4 / 2 0 0 3 1

2 : 1 5 : 2 1

2 9 / 0 4 / 2 0 0 3 1

2 : 3 2 : 4 5

2 9 / 0 4 / 2 0 0 3 1

2 : 4 2 : 4 1

2 9 / 0 4 / 2 0 0 3 1

2 : 5 2 : 3 7

2 9 / 0 4 / 2 0 0 3 1

3 : 0 2 : 3 2

2 9 / 0 4 / 2 0 0 3 1

3 : 1 2 : 2 8

2 9 / 0 4 / 2 0 0 3 1

3 : 2 2 : 2 4

2 9 / 0 4 / 2 0 0 3 1

3 : 3 2 : 2 0

2 9 / 0 4 / 2 0 0 3 1

3 : 4 2 : 1 6

2 9 / 0 4 / 2 0 0 3 1

3 : 5 2 : 1 2

2 9 / 0 4 / 2 0 0 3 1

4 : 0 2 : 0 8

2 9 / 0 4 / 2 0 0 3 1

4 : 1 2 : 0 3

2 9 / 0 4 / 2 0 0 3 1

4 : 2 1 : 5 9

2 9 / 0 4 / 2 0 0 3 1

4 : 3 1 : 5 5

2 9 / 0 4 / 2 0 0 3 1

4 : 4 1 : 5 1

2 9 / 0 4 / 2 0 0 3 1

4 : 5 1 : 4 7

2 9 / 0 4 / 2 0 0 3 1

5 : 0 1 : 4 3

2 9 / 0 4 / 2 0 0 3 1

5 : 1 1 : 3 9

2 9 / 0 4 / 2 0 0 3 1

5 : 2 1 : 3 4

2 9 / 0 4 / 2 0 0 3 1

5 : 3 1 : 3 0

2 9 / 0 4 / 2 0 0 3 1

5 : 4 1 : 2 6

2 9 / 0 4 / 2 0 0 3 1

5 : 5 1 : 2 2

2 9 / 0 4 / 2 0 0 3 1

6 : 0 1 : 1 8

2 9 / 0 4 / 2 0 0 3 1

6 : 1 1 : 1 4

Time


Acoustic sand detector trendGel only detected - did not detectthe sand injected



In this test the acoustic detector was only able to detect the gel injected and could not detectthe presence of the sand in the gel.

In contrast the ER probe responded to the sand injections and measured the damage it did. As seen previously the magnitude of the response did vary considerably. In this test only 34nm of metal was lost from the probe. The sand injection took place over 70 minutes and thisequates to a metal loss rate of ~0.26 mm/y.

This is an important result because it can be concluded that the ER probe can measure thedamage due to the very fine sand that is known to be produced in these gas fields.

DISCUSSION

This following discussion and conclusions are based on the entire project and reflect theresults of experiments not reported in this paper.

Equipment Installation: Advantages & disadvantages

Two important lessons from this work are:

1. The choice of sensor location is very important2. Proper installation of the sensors is critical.

These are not new observations but this program has highlighted the importance of gettingthese issues correct.

For probe location the probes should be installed at points of high turbulence or where the flowbombards the flowline wall directly. Points such as elbows and restrictions (eg valves, chokes)are usually the best.

In this project, representatives from all the companies that supplied the instruments werepresent for installation, training and initial data collection. This was obviously done to ensurethe best possible installation and to prevent any potential controversy or unfairness.


The installation of this probe was relatively simple although, being an intrusive one doesrequire the use of a high pressure installation tool operated by trained personnel. In thisproject the probe had to be installed in an existing access fitting which was on a straight run of pipe. This was not ideal but was compensated for to some extent by the use of an angleheaded intrusive probe that acts as a bend, allowing the flow to impinge directly onto it.

Flexible UT Mats

The installation of these probes was straightforward and they were located at the idealposition, close to bends in the flowline.

A problem encountered with these probes on flowline 7 was that after some days the sensor strip became unstuck from the flowline. This cannot be detected visually by inspecting the



strip but is discovered when the sensor fails to record data. The detachment can be proved byapplying force to the probe against the flowline and making a measurement. With the force inplace data was collected.

This problem has been observed in other BP locations worldwide.

To overcome this problem, a clamp was designed and installed by Bodycote following whichgood data was obtained. Thus the use of such a clamp is recommended for this instrument.

Acoustic Sand Detector

The installation of this probe was simple and straightforward. The probe cannot be installedclose to the choke, since noise from the choke generates an unacceptably high backgroundsignal (noise).

The probe should be installed where sand particles may bombard the flowline wall with themost force such as after an elbow on a downward section of the flowline or after an elbow on along horizontal section. Ideally, the probe should not be installed after a series of bends inwhich the distances apart are shorts since this may result in the sand particles losing their

energy (velocity).

Following the installations the instrument should be calibrated for background noise at differentvelocities and also detection of sand of known size and quantity.

One disadvantage is that the system cannot determine what sand size is produced. However,if a particular platform performs “shakeouts” then it may be possible to correlate this data withthe signal generated.


The installation was relatively simple although one problem was that the transducers neededto be an exact angle and distance apart. The angle and distance needed to be exact within acouple of degrees and millimeters.

Sadly, no satisfactory results were obtained with this sensor under the conditions of theexperiments. Consequently this probe is not recommended for sand detection.

Conventional UT Probe

This probe is not intended for permanent installation. It was used in manual mode to confirmthat no major wall loss was occurring.

High Sensitivity UT Probe

The installation of this probe was straightforward. However, following installation the readingsbecame erratic due to the couplant drying out. This issue had been discussed with themanufacturer but it was suggested that this would not be a problem.



Whilst the probe may be more sensitive than a conventional UT probe this could not be provedhere. Due to the drying of the couplant gel and the movement of the clamping system due toflowline vibration it is concluded that this probe is not suitable for permanent installation in atropical oilfield environment.

Instrument Results

The primary characteristics of the monitoring instruments considered in this work were thesensitivity, reliability, response time, repeatability and general usefulness for monitoring in fieldapplications.

Sand detectors analysis


From all the data observed, this detector did not detect any sand for the entire project. Oneobservation was that even if the sand detector worked the software for monitoring needs to be

simpler to interpret.

Thus it is concluded that this instrument is not capable of detecting sand under the conditionsin this work.

Acoustic Sand Detector

This detector detected all of the 80-120 μm sized sand injected. For the 30-50 μm sized sandnot all of injections were detected when the flow velocity and/or the rate of sand injection waslow.

Whenever sand was detected the response time was very fast and essentially instantaneous.The instruments were reliable and needed no intervention or maintenance.

Some disadvantages of the instrument are the loss of sensitivity under low velocity conditionsor with viscous fluids (not seen in this work but from other studies) and the noise interferencefrom slug flow or severe choke noise.

Metal loss rate monitoring instruments


For the background metal loss rates the inconnel element showed lower rates than the carbonsteel. This was expected since the Inconnel element only erodes and does not corrode. For the experiments with the carbon steel element, flowline 7, as expected showed a higher corrosion rate than flowline 1A. This was due to the significantly high water content of flowline7. For the corrosion data the results were in good agreement with those from the corrosioncoupon data.



When sand was injected, the system proved to be extremely sensitive. An importantobservation was the detection of metal loss when 30-50 μm sized sand was injected at a ratethat the sand detectors did not detect.

For injections with both sand sizes, it was observed that although the sand was detected themagnitudes of the responses were not always the same for the same conditions. This led tothe conclusions that either not all the sand bombarded the sensor or it was an error within theinstrument. For the case in which not all the sand bombards the sensor this is very important

to note and highlights the importance of installing the probe in the correct location. Ideally, for erosion monitoring the best strategy is to use a flush mounted probe installed at an elbow.

When the corrosivity of the environment changed, as when the inhibitor was not injected for some days, the probe detected the increase in corrosion rate.

Two potential disadvantages of this probe are:

1. For carbon steel elements, metal loss can be detected but the mechanism cannot(erosion vs corrosion). To discriminate between these an additional probe with acorrosion resistant element to measure erosion only must be used.

2. The element used can be supplied in various thicknesses that offer a balance betweensensitivity and lifetime. The elements eventually wear out so these probes need to bereplaced at a frequency dependent on the environment and element thickness.

It is concluded that this probe is very useful for both detecting sand and, more importantly,indicating how much metal is being lost due to a combination of erosion, corrosion and erosioncorrosion.

Flexible UT Mats

From the results, the background data did not show any wall loss for any of the flowlines. The

scatter in the data often suggested that there was some metal loss but careful, long termmonitoring, indicated that this was not so.

The probe was stable but for the small amount of metal loss generated in the tests thedetection limit was not sensitive enough. During the sand injections and non injection of theinhibitor the probe showed no metal loss.

It is concluded that these probes cannot be used for short term sand detection or small lossesin wall thickness. They are however ideal for the detection of excessive wall loss and wouldprovide a good back up measure for other instruments. They are also well suited to locationsthat are difficult to access for inspection personnel.

High Sensitivity UT Probe

The hardware continually gave problems throughout the project and no useful data wasrecorded. It may be more sensitive than a conventional UT probe although this was notproved here. When used in the same way as a conventional UT probe the readings wereaccurate and reliable. However, since it cannot be installed permanently there is noadvantage to using this instrument over a cheaper, more reliable conventional one.



This UT system is not recommended for use in an oilfield environment.

Conventional UT Probe

As expected from this conventional UT probe the results were reliable, accurate, stable andrepeatable. However, no metal loss was detected for any of the flowlines because the totalloss was less than the sensitivity of the instrument.

This probe provided an important check that no significant damage was done to the flowlinesduring these trials. It is not designed for permanent installation.

CONCLUSIONS

1. The choice of location of probes for both sand detection and wall loss measurements iscritical. Factors such as flow regime and system geometry must be accounted for indeciding where to locate a probe.

2. Installation of all of the probes was relatively straightforward but care is required. In

these tests representatives from all of the manufacturers of probes were present toinstall their systems to avoid concerns relating to inadequate installation.

3. All of the monitoring instruments have their uses in the appropriate environment but notall of them are useful for an oil and gas application.

4. The ultrasonic sand detector did not work under any conditions and is notrecommended for further evaluation.

5. The sensitive UT metal loss probe worked well but is not suited for permanentinstallation and there is no advantage to using it over a cheaper, conventional UT probe.

6. The conventional UT probe worked well to provide the assurance that no significant wallloss occurred during the tests in this work. This probe is not recommended for permanent installation.

7. The flexible UT mat, metal loss sensors worked well after a clamp was fitted to ensurethey stayed in contact with the flowline. They were not sensitive enough to detect anymetal loss due to the sand injected in these tests and are not recommend for short termmetal loss monitoring. They do, however provide, an ideal, on-line method for thedetection of significant wall loss. They would make a good back-up for other moresensitive instruments. They are also ideally suited for monitoring locations that are

difficult to reach.

8. The acoustic detectors are reliable, sensitive detectors of sand and require little or nomaintenance although they must be calibrated carefully. They detected all of the 80-120 μm sized sand under all of the conditions tested and in many cases did this belowthe recommended limit of 0.1 lbs/mmscf. For the 30-50 μm sized sand, which iscommonly produced in these gas fields, the results were not as impressive and at lower velocities the sand was not detected. At medium velocities the sand was detected but



at higher quantities than 0.1 lbs/mmscf. It may be that little or no damage is done belowthe detection limit of these sensors and so this may not be a problem. However, datafrom the ER probe suggests that this is not the case (see below). At high velocities the30-50 μm sand was detected at the 0.1 lbs/mmscf limit. This probe is recommended for permanent installation for the monitoring of sand and other solids.

9. The ER probe performed very well and detected almost all of the sand injected in theexperiments, including the 30-50 μm size at low velocities. There were two issues with

this probe:

a. Some erroneous results were obtained which indicated an increase in wallthickness.

b. The magnitude of the signal was not always constant for the same conditions.This may have been due different flow patterns of the sand.

10. Some erroneous results were obtained and the magnitude of the detection signal isvariable.

11. Using a combination of the acoustic sand detector and ER probe it should be possible

to increase the production rate of gas in a safe way to avoid erosion problems.

12. At this time it is not possible to provide a generalised flow limit for flowline design andeach well and flowline must be considered on an individual basis. Further work will beundertaken to define more carefully the exact limits.

REFERENCES

1. Recommended Practice For Design and Installation of Offshore Production Platform PipingSystems”, API Recommended Practice 14E, 5th Edition, October 1991 (revised 2000).

2. J.W. Martin, BP Erosion Guidelines, Revision 2.1, 1999.3. P. Birchenough, S. Dawson, T. Lockett & McCarthy, “Simultaneous Erosion & Corrosion in

Multiphase flow”, NACE 7th Middle East Conference on corrosion, Bahrain, 1996.4. B. Mclaury, S. Shirazi, J. Shadley & E. Rybicki, “Parameters Affecting Flow Accelerated

Erosion & Erosion-Corrosion”, NACE Corrosion 1995, Orlando, Florida, USA.

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